2001 y reg transit pickup probs?

my pickup (single wheel) as a vibration when driving, its not coming up through the steering wheel so i dont think its the front. it only does it at 55-57mph when empty and from 40mph + when loaded. also it only does it when on the gas or have the clutch pressed changing gear. when i let the gas off (coast) its fine. everything seems straight at the rear. anyone whos had the same prob and fixed it let me know how. thanks